Remove getting needless list length (#12998)

* Remove getting needless list length

* Cleanup

* Update illib.fs

* List.isSingleItem -> List.isSingleton
Co-authored-by: NDon Syme <dsyme@users.noreply.github.com>

src/fsharp/AugmentWithHashCompare.fs

@@ -376,8 +376,9 @@ let mkUnionCompare g tcref (tycon: Tycon) =
         let dtree = TDSwitch(thise, cases, dflt, m) 
         mbuilder.Close(dtree, m, g.int_ty)
 
-    let expr = 
-        if ucases.Length = 1 then expr else
+    let expr =
+        if List.isSingleton ucases then expr else
+
         let tagsEqTested = 
             mkCond DebugPointAtBinding.NoneAtSticky m g.int_ty  
               (mkILAsmCeq g m thistage thattage)
@@ -437,8 +438,9 @@ let mkUnionCompareWithComparer g tcref (tycon: Tycon) (_thisv, thise) (_thatobjv
         let dtree = TDSwitch(thise, cases, dflt, m) 
         mbuilder.Close(dtree, m, g.int_ty)
 
-    let expr = 
-        if ucases.Length = 1 then expr else
+    let expr =
+        if List.isSingleton ucases then expr else
+
         let tagsEqTested = 
             mkCond DebugPointAtBinding.NoneAtSticky m g.int_ty  
               (mkILAsmCeq g m thistage thattage)
@@ -497,8 +499,9 @@ let mkUnionEquality g tcref (tycon: Tycon) =
         let dtree = TDSwitch(thise, cases, dflt, m) 
         mbuilder.Close(dtree, m, g.bool_ty)
         
-    let expr = 
-        if ucases.Length = 1 then expr else
+    let expr =
+        if List.isSingleton ucases then expr else
+
         let tagsEqTested = 
           mkCond DebugPointAtBinding.NoneAtSticky m g.bool_ty  
             (mkILAsmCeq g m thistage thattage)
@@ -559,8 +562,9 @@ let mkUnionEqualityWithComparer g tcref (tycon: Tycon) (_thisv, thise) thatobje
         let dtree = TDSwitch(thise, cases, dflt, m) 
         mbuilder.Close(dtree, m, g.bool_ty)
         
-    let expr = 
-        if ucases.Length = 1 then expr else
+    let expr =
+        if List.isSingleton ucases then expr else
+
         let tagsEqTested = 
           mkCond DebugPointAtBinding.NoneAtSticky m g.bool_ty  
             (mkILAsmCeq g m thistage thattage)

src/fsharp/CheckComputationExpressions.fs

@@ -326,9 +326,15 @@ let TcComputationExpression (cenv: cenv) env (overallTy: OverallTy) tpenv (mWhol
         |> dict
 
     /// Decide if the identifier represents a use of a custom query operator
-    let tryGetDataForCustomOperation (nm: Ident) = 
+    let tryGetDataForCustomOperation (nm: Ident) =
+        let isOpDataCountAllowed opDatas =
+            match opDatas with
+            | [_] -> true
+            | _ :: _ -> cenv.g.langVersion.SupportsFeature LanguageFeature.OverloadsForCustomOperations
+            | _ -> false
+
         match customOperationMethodsIndexedByKeyword.TryGetValue nm.idText with 
-        | true, opDatas when (opDatas.Length = 1 || (opDatas.Length > 0 && cenv.g.langVersion.SupportsFeature LanguageFeature.OverloadsForCustomOperations)) -> 
+        | true, opDatas when isOpDataCountAllowed opDatas -> 
             for opData in opDatas do
                 let opName, maintainsVarSpaceUsingBind, maintainsVarSpace, _allowInto, isLikeZip, isLikeJoin, isLikeGroupJoin, _joinConditionWord, methInfo = opData
                 if (maintainsVarSpaceUsingBind && maintainsVarSpace) || (isLikeZip && isLikeJoin) || (isLikeZip && isLikeGroupJoin) || (isLikeJoin && isLikeGroupJoin) then 
@@ -1907,10 +1913,10 @@ let TcSequenceExpression (cenv: cenv) env tpenv comp (overallTy: OverallTy) m =
             //
             // This transformation is visible in quotations and thus needs to remain.
             | (TPat_as (TPat_wild _, PBind (v, _), _), 
-                vs, 
+                [_],
                 DebugPoints(Expr.App (Expr.Val (vf, _, _), _, [genEnumElemTy], [yieldExpr], _mYield), recreate)) 
-                    when vs.Length = 1 && valRefEq cenv.g vf cenv.g.seq_singleton_vref ->
-                    
+                    when valRefEq cenv.g vf cenv.g.seq_singleton_vref ->
+
                 // The debug point mFor is attached to the 'map'
                 // The debug point mIn is attached to the lambda
                 // Note: the 'yield' part of the debug point for 'yield expr' is currently lost in debug points. 

src/fsharp/CheckDeclarations.fs

@@ -4658,7 +4658,7 @@ module TcDeclarations =
             let _, tcref =
                 match ResolveTypeLongIdent cenv.tcSink cenv.nameResolver ItemOccurence.Binding OpenQualified envForDecls.NameEnv ad longPath resInfo PermitDirectReferenceToGeneratedType.No with
                 | Result res -> res
-                | res when inSig && longPath.Length = 1 ->
+                | res when inSig && List.isSingleton longPath ->
                     errorR(Deprecated(FSComp.SR.tcReservedSyntaxForAugmentation(), m))
                     ForceRaise res
                 | res -> ForceRaise res

src/fsharp/CheckExpressions.fs

@@ -903,9 +903,10 @@ let TcFieldInit (_m: range) lit = ilFieldToTastConst lit
 // Adjust the arities that came from the parsing of the toptyp (arities) to be a valSynData.
 // This means replacing the "[unitArg]" arising from a "unit -> ty" with a "[]".
 let AdjustValSynInfoInSignature g ty (SynValInfo(argsData, retData) as sigMD) =
-    if argsData.Length = 1 && argsData.Head.Length = 1 && isFunTy g ty && typeEquiv g g.unit_ty (domainOfFunTy g ty) then
+    match argsData with
+    | [[_]] when isFunTy g ty && typeEquiv g g.unit_ty (domainOfFunTy g ty) ->
         SynValInfo(argsData.Head.Tail :: argsData.Tail, retData)
-    else
+    | _ ->
         sigMD
 
 /// The ValReprInfo for a value, except the number of typars is not yet inferred

src/fsharp/ConstraintSolver.fs

@@ -1699,10 +1699,10 @@ and SolveMemberConstraint (csenv: ConstraintSolverEnv) ignoreUnresolvedOverload
                           | "?>="  | "?>"  | "?<="  | "?<"  | "?="  | "?<>" 
                           | ">=?"  | ">?"  | "<=?"  | "<?"  | "=?"  | "<>?" 
                           | "?>=?" | "?>?" | "?<=?" | "?<?" | "?=?" | "?<>?" ->
-                             if tys.Length = 1 then FSComp.SR.csTypeDoesNotSupportOperatorNullable(tyString, opName)
+                             if List.isSingleton tys then FSComp.SR.csTypeDoesNotSupportOperatorNullable(tyString, opName)
                              else FSComp.SR.csTypesDoNotSupportOperatorNullable(tyString, opName)
-                          | _ -> 
-                             if tys.Length = 1 then FSComp.SR.csTypeDoesNotSupportOperator(tyString, opName)
+                          | _ ->
+                             if List.isSingleton tys then FSComp.SR.csTypeDoesNotSupportOperator(tyString, opName)
                              else FSComp.SR.csTypesDoNotSupportOperator(tyString, opName)
                       return! ErrorD(ConstraintSolverError(err, m, m2))
 

src/fsharp/MethodOverrides.fs

@@ -164,7 +164,7 @@ module DispatchSlotChecking =
         match vsl with 
         | [thisv] :: vs -> 
             // Check for empty variable list from a () arg
-            let vs = if vs.Length = 1 && argInfos.IsEmpty then [] else vs
+            let vs = if List.isSingleton vs && argInfos.IsEmpty then [] else vs
             let implKind = 
                 if isInterfaceTy g implty then 
                     CanImplementAnyInterfaceSlot 

src/fsharp/NameResolution.fs

@@ -1246,7 +1246,7 @@ and private CanAutoOpenTyconRef (g: TcGlobals) m (tcref: TyconRef) =
     g.langVersion.SupportsFeature LanguageFeature.OpenTypeDeclaration &&
     not tcref.IsILTycon &&
     TryFindFSharpBoolAttribute g g.attrib_AutoOpenAttribute tcref.Attribs = Some true &&
-    tcref.Typars(m).Length = 0
+    tcref.Typars(m) |> List.isEmpty
 
 /// Add any implied contents of a type definition to the environment.
 and private AddPartsOfTyconRefToNameEnv bulkAddMode ownDefinition (g: TcGlobals) amap ad m  nenv (tcref: TyconRef) =

src/fsharp/Optimizer.fs

@@ -3314,7 +3314,7 @@ and StripPreComputationsFromComputedFunction g f0 args mkApp =
             fs, (remake >> (fun innerExprR -> Expr.DebugPoint (dp, innerExprR)))
 
         | _ -> 
-            [f], (fun newExprs -> (assert (newExprs.Length = 1)); List.head newExprs)
+            [f], (fun newExprs -> (assert (List.isSingleton newExprs)); List.head newExprs)
 
     match strip f0 with 
     | [f], remake -> 

src/fsharp/QuotationTranslator.fs

@@ -722,7 +722,7 @@ and private ConvExprCore cenv (env : QuotationTranslationEnv) (expr: Expr) : QP.
             | Some witnessArgIdx -> 
         
                 let witnessR = QP.mkVar witnessArgIdx
-                let args = if args.Length = 0 then [ mkUnit g m ] else args
+                let args = if List.isEmpty args then [ mkUnit g m ] else args
                 let argsR = ConvExprs cenv env args
                 (witnessR, argsR) ||> List.fold (fun fR argR -> QP.mkApp (fR, argR))
         

src/fsharp/TypedTreeOps.fs

@@ -2851,7 +2851,7 @@ module SimplifyTypes =
             simplify &&
             isTTyparCoercesToType tpc && 
             Zset.contains tp singletons && 
-            tp.Constraints.Length = 1)
+            List.isSingleton tp.Constraints)
         let inplace = inplace |> List.map (function tp, TyparConstraint.CoercesTo(ty, _) -> tp, ty | _ -> failwith "not isTTyparCoercesToType")
         
         { singletons = singletons
@@ -9269,23 +9269,29 @@ type Entity with
         tycon.TypeContents.tcaug_adhoc 
         |> NameMultiMap.find nm
         |> List.exists (fun vref -> 
-                          match vref.MemberInfo with 
-                          | None -> false 
-                          | Some membInfo -> 
-                                         let argInfos = ArgInfosOfMember g vref 
-                                         argInfos.Length = 1 && 
-                                         List.lengthsEqAndForall2 (typeEquiv g) (List.map fst (List.head argInfos)) argtys &&  
-                                         membInfo.MemberFlags.IsOverrideOrExplicitImpl) 
+            match vref.MemberInfo with 
+            | None -> false 
+            | Some membInfo ->
+ 
+            let argInfos = ArgInfosOfMember g vref 
+            match argInfos with
+            | [argInfos] ->
+                List.lengthsEqAndForall2 (typeEquiv g) (List.map fst argInfos) argtys &&  
+                membInfo.MemberFlags.IsOverrideOrExplicitImpl
+            | _ -> false) 
     
     member tycon.HasMember g nm argtys = 
         tycon.TypeContents.tcaug_adhoc 
         |> NameMultiMap.find nm
         |> List.exists (fun vref -> 
-                          match vref.MemberInfo with 
-                          | None -> false 
-                          | _ -> let argInfos = ArgInfosOfMember g vref 
-                                 argInfos.Length = 1 && 
-                                 List.lengthsEqAndForall2 (typeEquiv g) (List.map fst (List.head argInfos)) argtys) 
+            match vref.MemberInfo with 
+            | None -> false 
+            | _ ->
+
+            let argInfos = ArgInfosOfMember g vref
+            match argInfos with
+            | [argInfos] -> List.lengthsEqAndForall2 (typeEquiv g) (List.map fst argInfos) argtys
+            | _ -> false) 
 
 
 type EntityRef with 

src/fsharp/absil/illib.fs

@@ -476,6 +476,11 @@ module List =
         | [] -> true
         | h::t -> t |> List.forall (fun h2 -> h = h2)
 
+    let isSingleton xs =
+        match xs with
+        | [_] -> true
+        | _ -> false
+
 module ResizeArray =
 
     /// Split a ResizeArray into an array of smaller chunks.

src/fsharp/absil/illib.fsi

@@ -208,6 +208,8 @@ module internal List =
 
     val internal allEqual: xs:'T list -> bool when 'T: equality
 
+    val isSingleton: xs: 'T list -> bool
+
 module internal ResizeArray =
 
     /// Split a ResizeArray into an array of smaller chunks.

src/fsharp/infos.fs

@@ -2158,8 +2158,9 @@ type PropInfo =
         | FSProp(g, _, Some vref, _)  ->
             // A getter has signature  { OptionalObjectType } -> Unit -> PropertyType
             // A getter indexer has signature  { OptionalObjectType } -> TupledIndexerArguments -> PropertyType
-            let arginfos = ArgInfosOfMember g vref
-            arginfos.Length = 1 && arginfos.Head.Length >= 1
+            match ArgInfosOfMember g vref with
+            | [argInfos] -> not (List.isEmpty argInfos)
+            | _ -> false
         | FSProp(g, _, _, Some vref) ->
             // A setter has signature  { OptionalObjectType } -> PropertyType -> Void
             // A setter indexer has signature  { OptionalObjectType } -> TupledIndexerArguments -> PropertyType -> Void

src/fsharp/symbols/Exprs.fs

@@ -517,7 +517,7 @@ module FSharpExprConvert =
                 // type.  There is no witness for this case.  This is due to the code
                 //    let inline HashChar (x:char) = (# "or" (# "shl" x 16 : int #) x : int #)
                 // in FSharp.Core. 
-                | ErrorResult _  when vref.LogicalName =  "op_LeftShift" && tyargs.Length = 1 -> []
+                | ErrorResult _  when vref.LogicalName = "op_LeftShift" && List.isSingleton tyargs -> []
                 | res -> CommitOperationResult res
             let env = { env with suppressWitnesses = true }
             witnessExprs |> List.map (fun arg ->